Printed circuit boards ("PCBs") with electrical components are used in computers, communications equipment, televisions, and many other products. In typical applications, through-hole mounting and surface mounting techniques are used to attach electrical components to the top and/or bottom surfaces of PCBs. Since the electronics manufacturing industry is highly competitive, it is important to maximize the throughput of PCB processing and to securely attach the electrical components to the PCBs.
To surface mount components to a PCB, a solder and/or an adhesive compound is deposited onto the surfaces of the PCB, and then the components are pressed against the solder/adhesive. The solder/adhesive is preferably deposited onto the PCB with a highly accurate stenciling machine. Conventional stenciling machines generally have a stencil plate, a moveable work platform under the stencil plate, and a PCB holder attached to the platform. Conventional stenciling machines also have two spaced-apart, parallel conveyor tracks that transport the PCB along a conveyor line below the stencil plate but above the moveable platform.
In operation, the conveyor tracks transport the PCB until it is over the PCB holder, and then the work platform moves upwardly to engage the PCB holder with the bottom side of the PCB and lift the PCB to the stencil plate. Once the PCB is pressed against the stencil plate, a wiper moves across the stencil plate to press a solder or an adhesive through holes in the stencil plate and onto the top surface of the PCB. The work platform then moves downwardly away from the bottom surface of the stencil plate to replace the PCB on the conveyor rails.
Although conventional stenciling machines work well for processing large quantities of PCBs with a single size and shape, they are not well suited for contract PCB manufacturing in which five or more runs of PCBs with different sizes and shapes are processed in a single day. One problem with conventional PCB stenciling machines is that conventional PCB holders typically have a single size and shape to hold a single type of PCB. As a result, to adapt conventional stenciling machines to print on PCBs with different sizes and shapes, custom PCB assembly lines are typically shut down several times each day to replace one PCB holder with another having a different size. It will be appreciated that the downtime to change out the PCB holders significantly reduces the throughput of a PCB assembly line. Therefore, it would be desirable to develop an apparatus that reduces downtime in stenciling a solder or an adhesive in contract PCB manufacturing.
Another problem with conventional PCB stenciling machines is that the top surface of the PCB should be very flat as it presses against the bottom side of the stenciling plate to ensure that an adequate quantity of solder or adhesive is deposited uniformly across the PCB. To hold the PCB flat during the stenciling process, the PCB is generally attached to the PCB holder by a vacuum drawn against the bottom side of a PCB. Although holding the PCB to the PCB holder with a vacuum is desirable, only fixed-sized PCB holders are currently used with vacuum mounting systems to ensure that an adequate vacuum is maintained against the bottom side of a PCB. Therefore, it would also be desirable to develop a vacuum mounting apparatus that reduces downtime in stenciling a solder and an adhesive in contract PCB manufacturing.